Honeycomb structure

ABSTRACT

There is disclosed a honeycomb structure. A honeycomb structure includes a pillar-shaped honeycomb structure body having partition walls defining a plurality of cells which become through channels for a fluid and extend from a first end face to a second end face, the partition walls are constituted of a porous body having aggregates and a bonding material to bond the aggregates to one another in a state where pores are formed among the aggregates, the aggregates include molten silica particles, the bonding material includes glass, a content ratio of SiO 2  in the porous body is 70 mass % or more, and a thermal expansion coefficient of the porous body at 40 to 800° C. is from 1.5 to 6.0×10 −6 /° C.

The present application is an application based on JP2014-63841 filedwith Japan Patent Office on Mar. 26, 2014, the entire contents of whichare incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a honeycomb structure, and moreparticularly, it relates to a honeycomb structure for use to purify anexhaust gas emitted from a diesel engine.

2. Background Art

A technology to purify an exhaust gas of a gasoline engine is based onan engine operated at a stoichiometric air-fuel ratio and a technologyof a three-way catalyst to simultaneously purify HC, CO and NO_(x). Onthe other hand, a diesel engine is operated in oxygen excessiveatmosphere, and hence the abovementioned three-way catalyst does notfunction and NO_(x) cannot be reduced in oxygen excessive atmosphere.

An example of a technology to reduce NO_(x) in oxygen excessiveatmosphere is a technology of selective catalytic reduction (SCR). Inthe selective catalytic reduction (SCR), NO_(x) is reduced by usingammonia as a reducing agent. This technology is developed as atechnology to purify an exhaust gas from a stationary emission sourcesuch as a power generating plant, and a titania-vanadia based catalysthas been used. Hereinafter, the catalyst for use in the selectivecatalytic reduction will be referred to as the “SCR catalyst” sometimes.“SCR” is an abbreviation for “Selective Catalytic Reduction”.

The highly effective purification of NO_(x) emitted from the dieselengine is required and hence the technology to purify the exhaust gasfrom the stationary emission source is investigated to apply to a dieselvehicle. There are developed a technology to load the titania-vanadiabased catalyst to a cordierite honeycomb in the same manner as in thethree-way catalyst (Patent Document 1) and a technology to form thetitania-vanadia based catalyst in the honeycomb structure in the samemanner as in the catalyst for the stationary emission source (PatentDocument 2).

[Patent Document 1] JP-B-H08-11194

[Patent Document 2] JP-B-2675321

SUMMARY OF THE INVENTION

As an SCR catalyst, a “catalyst including titanium oxide as a maincomponent and further including tungsten oxide and vanadium oxide” asthe other catalyst components has attracted attention. Such a catalysthas a problem that it is difficult to inhibit the catalyst from peelingoff from partition walls, even when such a technology to adjust anamount of the catalyst to be loaded, particle diameters of the catalystand an open frontal area or fine pore volume of the partition wallsconstituting a honeycomb structure as described in Patent Document 1 isused. A reason for the problem is as follows. First, a thermal expansioncoefficient of the conventional honeycomb structure constituted ofcordierite is about 0.08×10⁻⁶/° C. (550° C.). On the other hand, thethermal expansion coefficient of the honeycomb structure coated with the“catalyst including titanium oxide as the main component and furtherincluding tungsten oxide and vanadium oxide” is 0.35×10⁻⁶/° C. (550° C.)and the thermal expansion coefficient noticeably varies before and afterthe honeycomb structure is coated with the above catalyst. That is, whenthe honeycomb structure is coated with the above catalyst, the thermalexpansion coefficient of the honeycomb structure constituted ofcordierite increases. This is supposedly because the thermal expansioncoefficient of the above catalyst has a higher value (specifically,6.0×10⁻⁶/° C., 550° C.) than the thermal expansion coefficient ofcordierite. Therefore, even when such a technology as described inPatent Document 1 is used for the use of the above catalyst, there is alimit to the inhibition of the catalyst from peeling off from thepartition walls. In consequence, there is highly demanded development ofan inventive technology capable of effectively inhibiting the catalystfrom peeling off from the partition walls, even if the above catalysthaving a comparatively high thermal expansion coefficient is loaded.

The present invention has been developed in view of the abovementionedproblem, and an object thereof is to provide a honeycomb structure whichis suitably usable as a catalyst support or a filter for use to purifyan exhaust gas emitted from an internal combustion engine such as adiesel engine or each type of combustion device. In particular, anobject of the present invention is to provide a honeycomb structure inwhich a catalyst can effectively be inhibited from peeling off frompartition walls, even if a catalyst having a comparatively high thermalexpansion coefficient as in a catalyst including titanium oxide as amain component and further including tungsten oxide and vanadium oxideis loaded.

To achieve the abovementioned objects, according to the presentinvention, there is provided a honeycomb structure as follows.

[1] A honeycomb structure including a pillar-shaped honeycomb structurebody having partition walls defining a plurality of cells which becomethrough channels for a fluid and extend from a first end face to asecond end face, wherein the partition walls are constituted of a porousbody having aggregates and a bonding material to bond the aggregates toone another in a state where pores are formed among the aggregates, theaggregates include molten silica particles, the bonding materialincludes glass, a content ratio of SiO₂ in the porous body is 70 mass %or more, and a thermal expansion coefficient of the porous body at 40 to800° C. is from 1.5 to 6.0×10⁻⁶/° C.

[2] The honeycomb structure according to the above [1], wherein porosityof the partition walls is from 10 to 60%.

[3] The honeycomb structure according to the above [1] or [2], wherein amass ratio of an alkali metal included in the porous body is 10 mass %or less.

[4] The honeycomb structure according to any one of the above [1] to[3], which further includes a catalyst loaded onto at least one of eachsurface of the partition walls and each pore formed in the partitionwalls, and including titanium oxide as a main component and furtherincluding tungsten oxide and vanadium oxide.

In a honeycomb structure of the present invention, partition walls of ahoneycomb structure body are constituted of a porous body havingaggregates and a bonding material to bond the aggregates to one anotherin a state where pores are formed among the aggregates. Furthermore, inthis porous body, the aggregates include molten silica particles and thebonding material includes glass. In the honeycomb structure of thepresent invention, a content ratio of SiO₂ in the porous body is 70 mass% or more and a thermal expansion coefficient of this porous body at 40to 800° C. is from 1.5 to 6.0×10⁻⁶/° C. The honeycomb structure of thepresent invention is suitably usable as a catalyst support or a filterfor use to purify an exhaust gas emitted from an internal combustionengine such as a diesel engine or each type of combustion device. Inparticular, the honeycomb structure of the present invention has ahigher thermal expansion coefficient of the honeycomb structure than aconventional honeycomb structure constituted of cordierite. Therefore,also when a catalyst having a comparatively high thermal expansioncoefficient as in a catalyst including titanium oxide as a maincomponent and including tungsten oxide and vanadium oxide is loaded, thecatalyst can effectively be inhibited from peeling off from thepartition walls caused by a temperature change. Furthermore, because an“effect of inhibiting the catalyst from peeling off” in the presentinvention noticeably comes from the thermal expansion coefficient in thematerial of the porous body constituting the partition walls, athickness of the partition walls, porosity of the partition walls andthe like are less restricted, and a degree of freedom in design of thehoneycomb structure increases.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic perspective view of one embodiment of a honeycombstructure of the present invention seen from an inflow end face side;

FIG. 2 is a schematic plan view of the honeycomb structure shown in FIG.1 and seen from the inflow end face side;

FIG. 3 is a schematic sectional view showing a cross section of thehoneycomb structure shown in FIG. 1 which is parallel to a cellextending direction;

FIG. 4 is an enlarged schematic view schematically showing a porous bodyconstituting partition walls;

FIG. 5 is a schematic perspective view of another embodiment of thehoneycomb structure of the present invention seen from an inflow endface side;

FIG. 6 is a schematic plan view of the honeycomb structure shown in FIG.5 and seen from the inflow end face side; and

FIG. 7 is a schematic sectional view showing a cross section of thehoneycomb structure shown in FIG. 5 which is parallel to a cellextending direction.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Next, a mode for carrying out the present invention will be described indetail with reference to the drawings. However, it should be understoodthat the present invention is not limited to the following embodimentsand that a change, an improvement or the like of design is suitablyadded on the basis of ordinary knowledge of a person skilled in the artwithout departing from the gist of the present invention.

(1) Honeycomb Structure:

One embodiment of a honeycomb structure of the present invention is ahoneycomb structure 100 including a pillar-shaped honeycomb structurebody 4 having partition walls 1 defining a plurality of cells 2 whichbecome through channels for a fluid and extend from a first end face 11to a second end face 12 as shown in FIG. 1 to FIG. 3. In the honeycombstructure 100 of the present embodiment, as shown in FIG. 4, thepartition walls 1 constituting the honeycomb structure body 4 areconstituted of a porous body 10 having aggregates 7 and a bondingmaterial 8 to bond the aggregates 7 to one another in a state wherepores 9 are formed among the aggregates 7. Furthermore, in the honeycombstructure 100 of the present embodiment, the aggregates 7 include moltensilica particles and the bonding material 8 includes glass. In thehoneycomb structure 100 shown in FIG. 1 to FIG. 3, the partition walls 1of the honeycomb structure body 4 are formed of the porous body 10 (seeFIG. 4) in which the “molten silica particles” as the aggregates 7 (seeFIG. 4) are bonded by using the “glass” as the bonding material 8 (seeFIG. 4). In the honeycomb structure 100 of the present embodiment, acontent ratio of SiO₂ in the porous body constituting the partitionwalls 1 is 70 mass % or more and a thermal expansion coefficient of theporous body at 40 to 800° C. is from 1.5 to 6.0×10⁻⁶/° C.

The honeycomb structure 100 is suitably usable as a catalyst support ora filter for use to purify an exhaust gas emitted from a diesel engine.Particularly, in the honeycomb structure 100 of the present embodiment,the thermal expansion coefficient of the porous body at 40 to 800° C. isfrom 1.5 to 6.0×10⁻⁶/° C., and the partition walls 1 (i.e., the porousbody 10 (see FIG. 4)) have a higher thermal expansion coefficient, ascompared with a conventional honeycomb structure constituted ofcordierite. Therefore, even when a catalyst having a comparatively highthermal expansion coefficient as in a catalyst including titanium oxideas a main component and further including tungsten oxide and vanadiumoxide is loaded, the catalyst can effectively be inhibited from peelingoff from the partition walls 1 caused by a temperature change.Furthermore, in the honeycomb structure 100, a thickness of thepartition walls 1, porosity of the partition walls 1 and the like areless restricted, and a degree of freedom in design of the honeycombstructure 100 increases. Hereinafter, in the present description, whenthe “thermal expansion coefficient” is simply mentioned, the thermalexpansion coefficient at 40 to 800° C. is meant unless otherwisespecifically noted.

Here, FIG. 1 is a schematic perspective view of one embodiment of thehoneycomb structure of the present invention seen from an inflow endface side. FIG. 2 is a schematic plan view of the honeycomb structureshown in FIG. 1 and seen from the inflow end face side. FIG. 3 is aschematic sectional view showing a cross section of the honeycombstructure shown in FIG. 1 which is parallel to a cell extendingdirection. FIG. 4 is an enlarged schematic view schematically showingthe porous body constituting the partition walls. The honeycombstructure 100 shown in FIG. 1 to FIG. 3 further has a circumferentialwall 3 positioned at the outermost circumference of the honeycombstructure body 4.

The aggregates of the porous body constituting the partition wallsinclude the molten silica particles. When the molten silica particlesare used as the aggregates, an excessive rise of the thermal expansioncoefficient of the porous body can be inhibited. That is, the glass asthe bonding material has a higher thermal expansion coefficient thancordierite, but when the glass is only used, the thermal expansioncoefficient of the honeycomb structure excessively heightens sometimes.Furthermore, when the honeycomb structure only constituted of the glassis used as a catalyst carrier, strength excessively lowers sometimes.When the aggregates including the molten silica particles are used, thethermal expansion coefficient of the honeycomb structure has anappropriate value (i.e., from 1.5 to 6.0×10⁻⁶/° C.), and furthermore,the strength of the honeycomb structure is so sufficient that thehoneycomb structure is usable as the catalyst carrier. The molten silicaparticles are preferably particles in which a purity of SiO₂ is 95 mass% or more. In the honeycomb structure of the present embodiment, SiO₂ isincluded in the molten silica particles as the aggregates and the glassas the bonding material, and a mass ratio of SiO₂ included in the porousbody to a total mass of the porous body is 70 mass % or more. Asdescribed above, the thermal expansion coefficient of the molten silicaparticles is different from that of the glass, and hence the thermalexpansion coefficient of the porous body varies in accordance with aratio between SiO₂ included as a component constituting the moltensilica particles and SiO₂ included as a component constituting thebonding material. In the honeycomb structure of the present embodiment,the ratio between SiO₂ included as the component constituting the moltensilica particles and SiO₂ included as the component constituting thebonding material is preferably adjusted so that the thermal expansioncoefficient of the porous body is from 1.5 to 6.0×10⁻⁶/° C.

The content ratio of SiO₂ in the porous body can be measured byfluorescent X-ray analysis, gravimetric analysis, and ICP-atomicemission spectrometry.

A measuring method of the thermal expansion coefficient of the porousbody at 40 to 800° C. is as follows. First, a measurement sample havinga vertical size of 5 mm×a horizontal size of 5 mm×a length of 50 mm isprepared from the honeycomb structure body of the honeycomb structure.Hereinafter, a direction from one end toward the other end of a portionof the above measurement sample which has the length of 50 mm will bereferred to as a “length direction of the measurement sample” sometimes.This measurement sample is cut out from the honeycomb structure andprepared so that a cell expanding direction of the honeycomb structurebody becomes the length direction of the measurement sample. An averagethermal expansion coefficient of the prepared measurement sample at 40to 800° C. is measured with a differential detection type of thermaldilatometer.

Particles constituting the aggregates are preferably substantially themolten silica particles, but aggregates other than the molten silicaparticles may be included. For example, a mass ratio of the moltensilica particles to all the particles constituting the aggregates ispreferably 20 mass % or more. According to such a constitution, thethermal expansion coefficient of the partition walls (i.e., the porousbody) is suitably higher than that of cordierite or the like. Examplesof the aggregates other than the molten silica particles include clay(kaolin), potsherd, and aluminum titanate.

The partition walls preferably include 10 to 60 mass % of the glass tothe total mass of the aggregates and the bonding material. When theratio of the glass is excessively small, a function of bonding theaggregate particles does not sufficiently develop sometimes. On theother hand, when the ratio of the glass is excessively large, there is apossibility that deterioration of a chemical durability or an excessiverise of the thermal expansion coefficient is caused.

A median diameter of the molten silica particles as the aggregates ispreferably from 5 to 80 μm. When the median diameter of the moltensilica particles is excessively small, the porous body cannot be formed,and a dense body is unfavorably formed. On the other hand, when themedian diameter of the molten silica particles is excessively large, aformability of the porous body unfavorably deteriorates. The mediandiameter of the molten silica particles can be measured through a grainsize distribution by a laser diffraction/scattering method.

The bonding material of the porous body constituting the partition wallsincludes the glass. Examples of the glass include aluminosilicate glass,zirconia soda glass, soda glass, non-alkali glass, and borosilicateglass. When such a glass is used as the bonding material, the thermalexpansion coefficient of the honeycomb structure can be heightened. Inthe glass as the bonding material, glass frit is preferably used as araw material. That is, during manufacturing, the glass frit ispreferably used as the raw material of the bonding material, and theglass frit and the molten silica particles are preferably mixed toprepare a forming raw material, thereby preparing the honeycombstructure. It is easy to control a melting temperature of the glassfrit, and hence it becomes easy to set conditions such as a firingtemperature, when a honeycomb formed body constituted of the forming rawmaterial is fired. Furthermore, the glass frit is inexpensive and easilyavailable as compared with a conventional ceramic raw material toprepare cordierite or the like, so that manufacturing cost of thehoneycomb structure can be reduced.

A content ratio of SiO₂ in the porous body is 70 mass % or more andpreferably 80 mass % or more. According to such a constitution, thechemical durability is suitably high. In addition, the thermal expansioncoefficient of the porous body is from 1.5 to 6.0×10⁻⁶/° C., but ispreferably from 2.5 to 5.0×10⁻⁶/° C. According to such a constitution,even when the catalyst having a comparatively high thermal expansioncoefficient is loaded, a difference between the thermal expansioncoefficient of the partition walls and the thermal expansion coefficientof the catalyst becomes smaller, and the catalyst can effectively beinhibited from peeling off from the partition walls.

The porous body may include an alkali metal. The alkali metal is mainlyincluded in the glass as the bonding material sometimes. For example,when the glass as the bonding material includes Na, a softening point ofthe glass is effectively lowered. A mass ratio of the alkali metal to beincluded in the porous body may be, for example, from 0 to 10 mass %.The alkali metal to be included in the glass may be included as an oxideof Na₂O or K₂O.

The porous body may include a component other than the alkali metal(hereinafter referred to as the “other component”). The other componentis preferably included in the glass as the bonding material. Examples ofthe other component to be included in the glass include CaO, Al₂O₃, andB₂O₃. The porous body may include 10 mass % or less of CaO, the porousbody may include 20 mass % or less of Al₂O₃, and the porous body mayinclude 10 mass % or less of B₂O₃, which produces the effect that thesoftening point of the glass as the bonding material is lowered. Achemical composition (mass %) of the other component of the porous bodycan be measured by the fluorescent X-ray analysis, gravimetric analysisand ICP-atomic emission spectrometry.

A softening temperature of the glass as the bonding material ispreferably from 400 to 1000° C., further preferably from 500 to 900° C.,and especially preferably from 600 to 800° C. When the softeningtemperature of the glass as the bonding material is low, a firingtemperature during the preparation of the honeycomb structure can belowered and the manufacturing cost can be reduced, as compared with theconventional honeycomb structure constituted of cordierite or the like.However, when the softening temperature of the glass is excessively low,melting of the honeycomb structure easily occurs sometimes in a casewhere the honeycomb structure is used as an exhaust gas purifyingcatalyst carrier. When the softening temperature of the glass is in theabovementioned numeric range, the honeycomb structure is suitably usableas the catalyst carrier and can further easily be manufactured.

Porosity of the partition walls is preferably from 10 to 60%, furtherpreferably from 20 to 50%, and especially preferably from 30 to 40%. Theporosity of the partition walls is porosity of the porous bodyconstituting the partition walls of the honeycomb structure. When theporosity of the partition walls is smaller than 10%, a pressure loss ofthe honeycomb structure enlarges sometimes. When the porosity of thepartition walls is in excess of 60%, the partition walls of thehoneycomb structure easily become brittle and lacking sometimes. Theporosity of the partition walls can be measured in conformity with JIS R1655 by mercury porosimetry.

A median diameter (a central pore diameter) of the partition walls ispreferably from 1 to 30 μm, further preferably from 1 to 20 μm, andespecially preferably from 5 to 10 μm. When the median diameter of thepartition walls is smaller than 1 μm, the pressure loss of the honeycombstructure enlarges sometimes. When the median diameter of the partitionwalls is in excess of 30 μm, the partition walls of the honeycombstructure easily become brittle and lacking sometimes. The mediandiameter of the partition walls can be measured in conformity with JIS R1655 by the mercury porosimetry.

There is not any special restriction on a thickness of the partitionwalls of the honeycomb structure body, but the thickness is preferablyfrom 30 to 330 μm, further preferably from 50 to 270 μm, and especiallypreferably from 100 to 230 μm. When the thickness of the partition wallsis in such a range, a rise of the pressure loss can be inhibited whilekeeping the strength of the partition walls of the honeycomb structure.

There is not any special restriction on a cell density of the honeycombstructure body, but the cell density is preferably from 5 to 200cells/cm², further preferably from 15 to 160 cells/cm², and especiallypreferably from 30 to 100 cells/cm². When the cell density is in such arange, it is possible to improve a purifying efficiency in a case wherethe honeycomb structure is used as the catalyst carrier.

There is not any special restriction on a shape of each cell formed inthe honeycomb structure body. Here, the “cell shape” is the shape of thecells in a cross section of the honeycomb structure body which isperpendicular to the cell extending direction. Examples of the cellshape include a quadrangular shape, a hexagonal shape, an octagonalshape, and any combination of these shapes.

There is not any special restriction on a shape of the honeycombstructure body, and examples of the shape include a pillar shape (around pillar shape) in which a bottom surface is circular, a pillarshape in which a bottom surface is oval, and a pillar shape in which abottom surface has a polygonal shape (a quadrangular shape, apentangular shape, a hexagonal shape, a heptagonal shape, an octagonalshape or the like).

A length of the honeycomb structure body from the first end face to thesecond end face and a size of the cross section of the honeycombstructure body which is perpendicular to the cell extending directionmay suitably be selected so that an optimum purification performance canbe obtained, for example, when the honeycomb structure is used as theexhaust gas purifying catalyst carrier. For example, the length of thehoneycomb structure body from the first end face to the second end faceis preferably from 50 to 440 mm and further preferably from 100 to 360mm. An area of the cross section of the honeycomb structure body whichis perpendicular to the cell extending direction is preferably from 50to 440 mm² and further preferably from 100 to 360 mm².

A-axis compressive strength of the honeycomb structure is preferably 5MPa or more, further preferably 10 MPa or more, and especiallypreferably 15 MPa or more. Here, the A-axis compressive strength is acompressive strength (MPa) stipulated in JASO Standard M505-87 which isa car standard issued by the society of Automotive Engineers of Japan.The A-axis compressive strength can be measured by the following method.First, a sample (a small honeycomb structure) having a diameter of 25.4mm and a height of 25.4 mm is cut out from the honeycomb structure and acompression load is loaded in a through channel direction of the sample.A pressure when the sample is broken by gradually enlarging thecompression load is obtained as the “A-axis compressive strength (MPa)”.

As shown in FIG. 5 to FIG. 7, a honeycomb structure 200 may include apillar-shaped honeycomb structure body 34 and an exhaust gas purifyingcatalyst 35. Here, the catalyst 35 is loaded onto at least one of eachsurface of partition walls 31 and each pore formed in the partitionwalls 31. Hereinafter, when the catalyst 35 is loaded onto at least oneof each surface of the partition walls 31 and each pore formed in thepartition walls 31, it is simply described sometimes that the “catalyst35 is loaded onto the partition walls 31”. The honeycomb structure body34 has a constitution similar to that of the honeycomb structure body 4shown in FIG. 1 to FIG. 3. That is, the honeycomb structure body 34 hasthe partition walls 31 defining a plurality of cells 32 which becomethrough channels for a fluid and extend from a first end face 41 to asecond end face 42. Furthermore, the partition walls 31 constituting thehoneycomb structure body 34 are constituted of a porous body havingaggregates and a bonding material to bond the aggregates to one anotherin a state where the pores are formed among the aggregates, theaggregates include molten silica particles, and the bonding materialincludes glass. Here, FIG. 5 is a schematic perspective view of anotherembodiment of the honeycomb structure of the present invention seen froman inflow end face side. FIG. 6 is a schematic plan view of thehoneycomb structure shown in FIG. 5 and seen from the inflow end faceside. FIG. 7 is a schematic sectional view showing a cross section ofthe honeycomb structure shown in FIG. 5 which is parallel to a cellextending direction.

An example of the exhaust gas purifying catalyst 35 is a catalystincluding titanium oxide as a main component and further includingtungsten oxide and vanadium oxide. The catalyst including titanium oxideas the main component and further including tungsten oxide and vanadiumoxide is a catalyst having a comparatively high thermal expansioncoefficient. Therefore, when the catalyst is loaded onto partition wallsof a conventional honeycomb structure, there is a problem that thecatalyst easily peels off from the partition walls. In the presentinvention, the partition walls are constituted of the porous bodyincluding molten silica particles as the aggregates and including theglass as the bonding material, and hence a thermal expansion coefficientof the honeycomb structure can be heightened. Therefore, even when thecatalyst including titanium oxide as the main component and furtherincluding tungsten oxide and vanadium oxide is loaded as the catalyst 35onto the partition walls 31 as in the honeycomb structure 200 shown inFIG. 5 to FIG. 7, the catalyst 35 can effectively be inhibited frompeeling off from the partition walls 31.

In the honeycomb structure of the present embodiment, as the exhaust gaspurifying catalyst, for example, catalysts of the following (a) to (c)are suitably usable: (a) a composite metal oxide made of tungsten oxide,cerium oxide, titanium oxide and zirconium oxide; (b) a catalystcontaining a titania-zirconia type composite oxide and a metal; and (c)a catalyst of an oxide of V, Cr, Ni, Cu or the like loaded onto TiO₂. Athermal expansion coefficient of a catalyst layer constituted of theabove catalyst at 40 to 600° C. is generally about 6.0×10⁻⁶/° C.

There is not any special restriction on an amount of the catalyst to beloaded, and the amount of the catalyst required to purify the exhaustgas is preferably loaded. For example, the amount of the catalystincluding titanium oxide as the main component and further includingtungsten oxide and vanadium oxide to be loaded is preferably from 50 to500 g/L and further preferably from 100 to 400 g/L. When the amount ofthe catalyst to be loaded is 50 g/L or more, a catalyst functionsufficiently develops. When the amount of the catalyst to be loaded is500 g/L or less, the pressure loss of the honeycomb structure does notexcessively enlarge, and the rise of the manufacturing cost can beinhibited. It is to be noted that the “amount of the catalyst to beloaded” is a mass (g) of the catalyst to be loaded per unit volume (1 L)of the honeycomb structure.

(2) Manufacturing Method of Honeycomb Structure:

Next, a method of manufacturing the honeycomb structure of the presentembodiment will be described. When the honeycomb structure ismanufactured, a forming raw material including raw material powder whichbecomes the aggregates and raw material powder which becomes the bondingmaterial is first prepared. This forming raw material is a forming rawmaterial to prepare the partition walls (i.e., the porous body) of thehoneycomb structure body.

The powder of the molten silica particles is usable as the raw materialwhich becomes the aggregates. The glass frit is usable as the rawmaterial which becomes the bonding material. Furthermore, kaolinparticles are preferably added to the raw material powder. When thekaolin particles are added to the raw material powder, a fluidity of akneaded material prepared from the raw material powder is improved, anda forming operation can be easily performed. In addition, clay, potsherdor the like may be added to the raw material powder. When the clay isadded to the raw material powder, the fluidity of the kneaded materialis improved. Additionally, a dispersing medium or an additive mayfurther be added to the forming raw material, in addition to theabovementioned raw materials.

A median diameter of the molten silica particles as the raw materialpowder is preferably from 5 to 80 When the median diameter of the moltensilica particles is smaller than 5 μm, the porosity unfavorably lowers.When the median diameter of the molten silica particles is in excess of80 μm, the formability unfavorably deteriorates. The median diameter ofthe molten silica particles is a value measured by laser diffractometry.

A median diameter of the glass frit as the raw material powder ispreferably 80 μm or less. When the median diameter of the glass frit isin excess of 40 the formability unfavorably deteriorates. The mediandiameter of the glass fit is a value measured by the laserdiffractometry.

There is not any special restriction on kaolin and clay particles as theraw material powder, but a median diameter thereof is preferably 80 μmor less, further preferably 40 μm or less, and especially preferably 10μm or less. When the kaolin particles having such a median diameter areused, the fluidity of the kneaded material prepared from the rawmaterial powder suitably improves.

There is not any special restriction on a blend ratio of the rawmaterial powder, but a blend ratio of the molten silica particles ispreferably from 20 to 80 mass %, a blend ratio of the glass frit ispreferably from 20 to 80 mass %, and a blend ratio of the kaolinparticles is preferably from 10 to 50 mass %. It is to be noted that theabove blend ratios are set so that a total of the respective blendratios of the molten silica particles, the glass frit and the kaolinparticles is 100 mass %.

Examples of the additive include a binder and a pore former. An exampleof the dispersing medium is water.

Examples of the binder include methylcellulose, hydroxypropoxylcellulose, hydroxyethyl cellulose, carboxymethylcellulose, and polyvinylalcohol. There is not any special restriction on the pore former, aslong as the pore former becomes pores after fired, and examples of thepore former include starch, a foamable resin, a water absorbable resin,silica gel, and wood chips.

Particle diameters of the raw material powder and an amount of thepowder to be blended as well as particle diameters of powder of the poreformer to be added and an amount of the powder to be blended areregulated, so that the porous body having desirable porosity and mediandiameter can be obtained.

Next, the obtained forming raw material is kneaded to form a kneadedmaterial. There is not any special restriction on a method of formingthe kneaded material, and an example of the method is a method in whicha kneader, a vacuum pugmill or the like is used.

Next, the obtained kneaded material is extruded to prepare a honeycombformed body. The extrusion can be performed by using a die having adesirable cell shape, partition wall thickness and cell density. Next,the obtained honeycomb formed body may be dried to obtain a honeycombdried body prepared by drying the honeycomb formed body. There is notany special restriction on a drying method, but examples of the methodinclude hot air drying, microwave drying, dielectric drying,reduced-pressure drying, vacuum drying, and freeze drying. Among thesemethods, the dielectric drying, the microwave drying or the hot airdrying is preferably performed alone or any combination thereof ispreferably performed. Furthermore, as drying conditions, a dryingtemperature is preferably from 30 to 150° C. and a drying time ispreferably from one minute to two hours.

Next, the honeycomb formed body or the honeycomb dried body is fired.The obtained honeycomb fired body becomes the honeycomb structure of thepresent embodiment. A firing temperature is preferably from 400 to 1400°C. and further preferably from 600 to 1200° C. When the firingtemperature is lower than 400° C., the glass frit as the bondingmaterial is not sufficiently dissolved and the aggregates aredisadvantageously insufficiently bonded to one another. On the otherhand, when the firing temperature is in excess of 1400° C., cristobaliteis generated, and the thermal expansion coefficient excessively rises,or energy consumption of a furnace excessively enlarges sometimes.Furthermore, a firing time at the highest temperature is preferably fromabout 30 minutes to eight hours. The firing can be performed in, forexample, an air atmosphere, a steam atmosphere, or a hydrocarbon gascombustion atmosphere.

EXAMPLES

Hereinafter, the present invention will further specifically bedescribed in accordance with examples, but the present invention is notlimited to these examples.

Example 1

In Example 1, first, a forming raw material was prepared by using moltensilica particles, glass frit, and kaolin particles. As to blend ratiosof the respective components in the forming raw material, the ratio ofthe molten silica particles was 50 mass %, the ratio of the glass fritwas 30 mass %, and the ratio of the kaolin particles was 20 mass %. Amedian diameter of the molten silica particles was 15 μm, a mediandiameter of the glass frit was 10 μm, and a median diameter of thekaolin particles was 5 μm. As the glass frit, glass frit constituted ofaluminosilicate glass was used. The glass frit used in Example 1 was“aluminosilicate glass A”. A column of “glass type” of Table 1 shows thetype of glass frit used in preparing the forming raw material.Furthermore, a column of “clay” of Table 1 shows components other thanthe molten silica particles and glass frit used in preparing the formingraw material. In Example 1, the abovementioned kaolin particlescorresponded to this clay. A column of “blend ratio (mass %) of glass”of Table 1 shows the blend ratio of the glass frit. A column of “blendratio (mass %) of silica” of Table 1 shows the blend ratio of the moltensilica particles.

TABLE 1 Firing Blend ratio Blend ratio Blend ratio temp. of glass ofsilica of clay Porous body Glass type Clay (° C.) (mass %) (mass %)(mass %) Example 1 Molten silica particles + glass Aluminosilicate glassA Kaolin 1000 30 50 20 Example 2 Molten silica particles + glassAluminosilicate glass A Clay 1000 30 50 20 Example 3 Molten silicaparticles + glass Aluminosilicate glass B Kaolin 1000 30 50 20 Example 4Molten silica particles + glass Aluminosilicate glass A Kaolin 1000 4045 15 Example 5 Molten silica particles + glass Aluminosilicate glass BClay 1000 30 50 20 Example 6 Molten silica particles + glassAluminosilicate glass B Kaolin 1000 20 60 20 Example 7 Molten silicaparticles + glass Aluminosilicate glass B Kaolin 1000 10 70 20 Example 8Molten silica particles + glass Soda glass Clay 900 60 30 10 Example 9Molten silica particles + glass Soda glass Clay 700 60 30 10 Example 10Molten silica particles + glass Soda glass Clay 700 40 50 10 Example 11Molten silica particles + glass Soda glass Clay 1000 20 50 30 Example 12Molten silica particles + glass Soda glass Clay 900 20 50 30 Example 13Molten silica particles + glass Soda glass Clay 800 20 50 30 Example 14Molten silica particles + glass Soda glass Clay 700 20 50 30 ComparativeMolten silica particles + glass Aluminosilicate glass A Kaolin 1100 3050 20 Example 1 Comparative Molten silica particles + glassAluminosilicate glass A Kaolin 1200 30 50 20 Example 2 ComparativeMolten silica particles + glass Soda glass Clay 1000 60 30 10 Example 3Comparative Molten silica particles + glass Soda glass Clay 1000 40 5010 Example 4 Comparative Molten silica particles + glass Soda glassKaolin 1000 10 70 20 Example 5 Reference Solid type catalyst — — — — — —Example 1 Comparative Cordierite — — — — — — Example 6

Furthermore, chemical components of the “aluminosilicate glass A” usedas the glass fit were measured by gravimetric analysis and ICP-atomicemission spectrometry. Table 2 shows the chemical components of thealuminosilicate glass A.

TABLE 2 Aluminosilicate Aluminosilicate Soda Chemical components glass Aglass B glass SiO₂ (mass %) 61.9 61.4 71.6 Al₂O₃ (mass %) 17.8 19.5 1.3CaO (mass %) 7.6 0.1 8.1 Na₂O (mass %) 0.1 13.1 13.6 B₂O₃ (mass %) 8.92.7 0.0

In Example 1, the abovementioned molten silica particles, glass frit andkaolin particles were used, and additionally, a water absorbable resinas a pore former, methylcellulose as a binder and an appropriate amountof water were added.

Next, the obtained forming raw material was kneaded with a kneader andthen kneaded with a vacuum pugmill to form a kneaded material. Next, theobtained kneaded material was extruded to prepare a honeycomb formedbody. The honeycomb formed body was prepared so as to obtain a partitionwall thickness of 170 μm and a cell density of 62 cells/cm² after fired.Next, the honeycomb formed body was dried to obtain a honeycomb driedbody. As to the drying, microwave drying was first performed and thenhot air drying was performed. Next, the honeycomb dried body was cut sothat a length of the honeycomb dried body in a cell extending directionwas a predetermined length. Next, the obtained honeycomb dried body wasdegreased. The degreasing was performed at 400° C. in the air atmospherefor two hours. Next, the degreased honeycomb dried body was fired toobtain a honeycomb structure. The firing was performed at 1000° C. inthe air atmosphere for two hours. The obtained honeycomb structure had around pillar shape in which a diameter of each end face was 144 mm and alength in the cell extending direction was 153 mm. Furthermore, theobtained honeycomb structure had a circumferential wall having athickness of 0.52 mm at the outermost circumference of a honeycombstructure body. Additionally, partition walls of the obtained honeycombstructure were constituted of a porous body having molten silicaparticles as aggregates, and glass to bond the molten silica particlesto one another in a state where pores were formed among the moltensilica particles. As to the honeycomb structure having the partitionwalls constituted of such a porous body, a column of “porous body” ofTable 1 shows the “molten silica particles+the glass”.

As to the obtained honeycomb structure, a “porosity (%)”, a “thermalexpansion coefficient (×10⁻⁶/° C.)”, “A-axis compressive strength (MPa)”and a “median diameter (μm)” were measured by the following methods.Table 3 shows the measurement results.

[Porosity]

As the porosity (%), the porosity of the partition walls of thehoneycomb structure was measured by mercury porosimetry (JIS R 1655).

[Thermal Expansion Coefficient (×10⁻⁶/° C.)]

The thermal expansion coefficient (×10⁻⁶/° C.) was obtained by measuringan average thermal expansion coefficient (×10⁻⁶/° C.) of the porous bodyconstituting the partition walls at 40 to 800° C. with a differentialdetection type of thermal dilatometer. Specifically, a measurementsample having a vertical size of 5 mm×a horizontal size of 5 mm×a lengthof 50 mm was first prepared from the honeycomb structure body of thehoneycomb structure. A length direction of this measurement samplecorresponded to the cell extending direction of the honeycomb structurebody. Next, the average thermal expansion coefficient of the preparedmeasurement sample at 40 to 800° C. was measured with the differentialdetection type of thermal dilatometer.

[A-Axis Compressive Strength (MPa)]

First, a sample having a diameter of 25.4 mm and a height of 25.4 mm (asmall honeycomb structure) was cut out from the honeycomb structure, anda compression load was loaded in a through channel direction of thesample. A pressure when the sample was broken by gradually enlarging thecompression load was obtained as the “A-axis compressive strength(MPa)”.

[Median Diameter (μm)]

As the median diameter (μm), the median diameter of the partition wallsof the honeycomb structure was measured by the mercury porosimetry (JISR 1655).

Furthermore, chemical components of the honeycomb structure of Example 1were measured by fluorescent X-ray analysis, gravimetric analysis andICP-atomic emission spectrometry. Table 3 shows the chemical componentsof the honeycomb structure of Example 1.

TABLE 3 Thermal expansion A-axis Partition coefficient compressiveMedian wall Cell Porosity (40 to 800° C.) strength dia. thicknessdensity Dia. (%) (×10⁻⁶/° C.) (MPa) (μm) (μm) (cells/cm³) (mm) Example 145 2.7 5.8 1.7 170 62 144 Example 2 37 4.5 15.6 1.2 170 62 144 Example 318 3.7 12 2.1 170 62 144 Example 4 47 3.2 7.7 4.0 170 62 70 Example 5 453.7 5.5 3.2 170 62 70 Example 6 49 2.9 0.5 1.9 170 62 70 Example 7 521.9 0.5 1.3 170 62 70 Example 8 4.8 3.7 92 32 304 76 70 Example 9 31 4.51.7 0.4 304 76 70 Example 10 35 4.7 1.7 0.4 304 76 70 Example 11 24 3.124 0.9 170 62 70 Example 12 25 2.9 13 0.3 170 62 70 Example 13 25 2.76.7 0.2 170 62 70 Example 14 25 2.5 3.8 0.2 170 62 70 Comparative 46 6.48.3 7.2 170 62 70 Example 1 Comparative 40 11.1 13.9 9.6 170 62 70Example 2 Comparative 0.7 6.2 20 22 304 76 70 Example 3 Comparative 246.6 65 4.2 304 76 70 Example 4 Comparative 51 Unmeasurable 0.1 1.2 17062 70 Example 5 Reference 54 6.5 (to 600° C.) 9.6 0.03 304 50 144Example 1 Comparative 35 0.5 15 4.5 170 62 144 Example 6 Length SiO₂Al₂O₃ CaO Na₂O B₂O₃ (mm) (mass %) (mass %) (mass %) (mass %) (mass %)Example 1 153 81 13 2 0 3 Example 2 153 80 11 1 0 2 Example 3 153 80 130 4 1 Example 4 100 76 13 3 0 4 Example 5 100 80 12 0 2 0 Example 6 10081 12 0 3 1 Example 7 100 83 11 0 1 0 Example 8 100 79 4 5 8 0 Example 9100 79 4 5 8 0 Example 10 100 84 3 3 5 0 Example 11 100 81 9 2 3 0Example 12 100 81 9 2 3 0 Example 13 100 81 9 2 3 0 Example 14 100 81 92 3 0 Comparative 100 77 15 1 0 2 Example 1 Comparative 100 77 15 1 0 2Example 2 Comparative 100 79 4 5 8 0 Example 3 Comparative 100 84 3 3 50 Example 4 Comparative 100 84 9 1 1 0 Example 5 Reference 153 8 1 2 0 0Example 1 Comparative 153 55 31 0 0 0 Example 6

Example 2

In Example 2, a forming raw material was first prepared by using moltensilica particles, glass frit and clay. As to blend ratios of therespective components in the forming raw material, the ratio of themolten silica particles was 50 mass %, the ratio of the glass frit was30 mass %, and the ratio of the clay was 20 mass %. As the clay, Kibushiclay was used. In Example 2, the procedures of Example 1 were repeatedexcept that the forming raw material was prepared as described above, toprepare a honeycomb structure of Example 2. As to the obtained honeycombstructure, “porosity (%)”, a “thermal expansion coefficient (×10⁻⁶/°C.)”, “A-axis compressive strength (MPa)” and a “median diameter (μm)”were measured in the same manner as in Example 1. Table 3 shows themeasurement results. Furthermore, chemical components of the honeycombstructure of Example 2 were measured in the same manner as in Example 1.Table 3 shows the chemical components of the honeycomb structure ofExample 2.

Example 3

The procedures of Example 1 were repeated except that as glass frit,“aluminosilicate glass B” of such a chemical composition as shown inTable 2 was used, to prepare a honeycomb structure of Example 3. As tothe obtained honeycomb structure, “porosity (%)”, a “thermal expansioncoefficient (×10⁻⁶/° C.)”, “A-axis compressive strength (MPa)” and a“median diameter (μm)” were measured in the same manner as in Example 1.Table 3 shows the measurement results. Furthermore, chemical componentsof the honeycomb structure of Example 3 were measured in the same manneras in Example 1. Table 3 shows the chemical components of the honeycombstructure of Example 3.

Examples 4 to 7

The procedures of Example 1 were repeated except that a glass type,clay, a firing temperature and respective blend ratios were changed asshown in Table 1 and except that a partition wall thickness, a celldensity and a diameter were changed as shown in Table 3, to preparehoneycomb structures of Examples 4 to 7. As to each obtained honeycombstructure, “porosity (%)”, a “thermal expansion coefficient (×10⁻⁶/°C.)”, “A-axis compressive strength (MPa)” and a “median diameter (μm)”were measured in the same manner as in Example 1. Table 3 shows themeasurement results. Furthermore, chemical components of the honeycombstructures of Examples 4 to 7 were measured in the same manner as inExample 1. Table 3 shows the chemical components of the honeycombstructures of Examples 4 to 7.

Examples 8 to 14

The procedures of Example 1 were repeated except that a glass type,clay, a firing temperature and respective blend ratios were changed asshown in Table 1 and except that a partition wall thickness, a celldensity and a diameter were changed as shown in Table 3, to preparehoneycomb structures. In Examples 8 to 14, “soda glass” of such achemical composition as shown in Table 2 was used as glass fit. As toeach obtained honeycomb structure, “porosity (%)”, a “thermal expansioncoefficient (×10⁻⁶/° C.)”, “A-axis compressive strength (MPa)” and a“median diameter (μm)” were measured in the same manner as in Example 1.Table 3 shows the measurement results. Furthermore, chemical componentsof the honeycomb structures of Examples 8 to 14 were measured in thesame manner as in Example 1. Table 3 shows the chemical components ofthe honeycomb structures of Examples 8 to 14.

Comparative Examples 1 and 2

The procedures of Example 1 were repeated except that a firingtemperature was changed as shown in Table 1 and except that a partitionwall thickness, a cell density and a diameter were changed as shown inTable 3, to prepare honeycomb structures of Comparative Examples 1 and2. As to each obtained honeycomb structure, “porosity (%)”, a “thermalexpansion coefficient (×10⁻⁶/° C.)”, “A-axis compressive strength (MPa)”and a “median diameter (μm)” were measured in the same manner as inExample 1. Table 3 shows the measurement results. Furthermore, chemicalcomponents of the honeycomb structures of Comparative Examples 1 and 2were measured in the same manner as in Example 1. Table 3 shows thechemical components of the honeycomb structures of Comparative Examples1 and 2. In Comparative Example 1, the firing temperature was set to1100° C., and the thermal expansion coefficient thereby indicated a highvalue of 6.4×10⁻⁶/° C. In Comparative Example 2, the firing temperaturewas set to 1200° C., and the thermal expansion coefficient therebyindicated a high value of 11.1×10⁻⁶/° C. It is presumed that inComparative Examples 1 and 2, at least a part of molten silica particlesand glass was crystallized into cristobalite or the like during firingand the thermal expansion coefficient enlarged.

Comparative Examples 3 to 5

The procedures of Example 8 were repeated except that clay, a firingtemperature and respective blend ratios were changed as shown in Table 1and except that a partition wall thickness, a cell density and adiameter were changed as shown in Table 3, to prepare honeycombstructures of Comparative Examples 3 to 5. As to each obtained honeycombstructure, “porosity (%)”, a “thermal expansion coefficient (×10⁻⁶/°C.)”, “A-axis compressive strength (MPa)” and a “median diameter (μm)”were measured in the same manner as in Example 1. Table 3 shows themeasurement results. Furthermore, chemical components of the honeycombstructures of Comparative Examples 3 to 5 were measured in the samemanner as in Example 1. Table 3 shows the chemical components of thehoneycomb structures of Comparative Examples 3 to 5. Additionally, inComparative Example 5, the thermal expansion coefficient wasunmeasurable due to insufficient strength of the honeycomb structure. Acolumn of “thermal expansion coefficient” shows “unmeasurable” forComparative Example 5.

Reference Example 1

In Reference Example 1, a solid type catalyst was used. “Porosity (%)”,a “thermal expansion coefficient (×10⁻⁶/° C.)”, “A-axis compressivestrength (MPa)” and a “median diameter (μm)” were measured in the samemanner as in Example 1. Table 3 shows the measurement results.Furthermore, chemical components of the solid type catalyst of ReferenceExample 1 were measured in the same manner as in Example 1. Table 3shows the chemical components of the solid type catalyst of ReferenceExample 1. The solid type catalyst in Reference Example 1 includedvanadium oxide having a melting point of 675° C., and hence as thethermal expansion coefficient, the thermal expansion coefficient at 40to 600° C. was measured.

Comparative Example 6

In Comparative Example 6, a plurality of materials selected from a groupconsisting of talc, kaolin, calcinated kaolin, alumina, aluminumhydroxide and silica were combined to prepare a cordierite forming rawmaterial. The materials were blended at predetermined ratios so that achemical composition of the cordierite forming raw material included 42to 56 mass % of SiO₂, 0 to 45 mass % of Al₂O₃, and 12 to 16 mass % ofMgO. To the cordierite forming raw material prepared in this manner,graphite and a synthetic resin as pore formers were added as much as anamount required to obtain a targeted porosity. Furthermore, appropriateamounts of methylcellulose and a surfactant were added, respectively,and then water was added, followed by kneading, to prepare a kneadedmaterial. The prepared kneaded material was subjected to vacuumdeaeration and then extruded to obtain a honeycomb formed body. Thehoneycomb formed body was prepared so as to obtain a partition wallthickness of 170 μm and a cell density of 62 cells/cm² after fired.Next, the honeycomb formed body was dried to obtain a honeycomb driedbody. As to the drying, microwave drying was first performed and thenhot air drying was performed. Next, the honeycomb dried body was cut sothat a length of the honeycomb dried body in a cell extending directionbecame a predetermined length, and the honeycomb dried body wasdegreased. Next, the degreased honeycomb dried body was fired to obtaina honeycomb structure of Comparative Example 6. The firing was performedat 1410 to 1440° C. The obtained honeycomb structure had a round pillarshape in which a diameter of each end face was 144 mm and a length inthe cell extending direction was 153 mm. Furthermore, in the obtainedhoneycomb structure, a honeycomb structure body had a circumferentialwall having a thickness of 1.13 mm at the outermost circumference of thehoneycomb structure body. As to the obtained honeycomb structure,“porosity (%)”, a “thermal expansion coefficient (×10⁻⁶/° C.)”, “A-axiscompressive strength (MPa)” and a “median diameter (μm)” were measuredin the same manner as in Example 1. Table 3 shows the measurementresults. In addition, chemical components of the honeycomb structure ofComparative Example 6 were measured in the same manner as in Example 1.Table 3 shows the chemical components of the honeycomb structure ofComparative Example 6.

(Results)

In a honeycomb structure for use as an exhaust gas purifying catalystcarrier to purify an exhaust gas, a thermal expansion coefficient ispreferably as low as possible from the viewpoint of a thermal shockresistance. Specifically, from the viewpoint of the thermal shockresistance, the thermal expansion coefficient of the honeycomb structureis preferably 6.0×10⁻⁶/° C. or less. On the other hand, from theviewpoint of a peeling resistance of a catalyst layer constituted of acatalyst loaded onto the honeycomb structure, a difference in thermalexpansion coefficient between the honeycomb structure and the catalystlayer is preferably as small as possible. A honeycomb structure made ofcordierite having low thermal expansion as in the honeycomb structure ofComparative Example 6 is known for the problem that the catalyst layerpeels off, and has a thermal expansion coefficient of about 0.5 to1.0×10⁻⁶/° C. (0.5×10⁻⁶/° C. in Comparative Example 6). To avoid theabovementioned problem that the catalyst layer peels off, the thermalexpansion coefficient of the honeycomb structure is preferably1.5×10⁻⁶/° C. or more. The thermal expansion coefficient of the catalystlayer is about 6.0×10⁻⁶/° C. (in the solid type catalyst of ReferenceExample 1, 6.5×10⁻⁶/° C.). When the thermal expansion coefficient of thehoneycomb structure is in excess of the thermal expansion coefficient ofthe catalyst layer, both the thermal shock resistance and the peelingresistance worsen.

It is seen from the measurement results of the thermal expansioncoefficient shown in Table 3 that each of the honeycomb structures ofExamples 1 to 14 is more excellent in balance between the thermal shockresistance and the peeling resistance than each of the honeycombstructures of Comparative Examples 1 to 6. In the honeycomb structure ofComparative Example 3, porosity is low, and hence a catalyst retainingperformance is low. Therefore, even when the difference in thermalexpansion coefficient between the honeycomb structure and the catalystlayer is small, the catalyst layer easily peels off. The honeycombstructure of Comparative Example 5 has such a low mechanical strengththat a sample to measure the thermal expansion coefficient cannot be cutout therefrom, and the honeycomb structure cannot be resistant to actualuse.

A honeycomb structure of the present invention can be utilized as anexhaust gas purifying catalyst carrier to purify an exhaust gas.

DESCRIPTION OF REFERENCE NUMERALS

1 and 31: partition wall, 2 and 32: cell, 3 and 33: circumferentialwall, 4 and 34: honeycomb structure body, 7: aggregates, 8: bondingmaterial, 9: pore, 10: porous body, 11 and 41: first end face (endface), 12 and 42: second end face (end face), 35: catalyst, and 100 and200: honeycomb structure.

What is claimed is:
 1. A honeycomb structure comprising a pillar-shaped honeycomb structure body having partition walls defining a plurality of cells which become through channels for a fluid and extend from a first end face to a second end face, wherein the partition walls are constituted of a porous body having aggregates and a bonding material to bond the aggregates to one another in a state where pores are formed among the aggregates, the aggregates include molten silica particles, and the bonding material includes glass, and a content ratio of SiO₂ in the porous body is 70 mass % or more, and a thermal expansion coefficient of the porous body at 40 to 800° C. is from 1.5 to 6.0×10⁻⁶/° C.
 2. The honeycomb structure according to claim 1, wherein porosity of the partition walls is from 10 to 60%.
 3. The honeycomb structure according to claim 1, wherein a mass ratio of an alkali metal included in the porous body is from 10 mass % or less.
 4. The honeycomb structure according to claim 2, wherein a mass ratio of an alkali metal included in the porous body is from 10 mass % or less.
 5. The honeycomb structure according to claim 1, which further comprises a catalyst loaded onto at least one of each surface of the partition walls and each pore formed in the partition walls, and including titanium oxide as a main component and further including tungsten oxide and vanadium oxide.
 6. The honeycomb structure according to claim 2, which further comprises a catalyst loaded onto at least one of each surface of the partition walls and each pore formed in the partition walls, and including titanium oxide as a main component and further including tungsten oxide and vanadium oxide.
 7. The honeycomb structure according to claim 3, which further comprises a catalyst loaded onto at least one of each surface of the partition walls and each pore formed in the partition walls, and including titanium oxide as a main component and further including tungsten oxide and vanadium oxide.
 8. The honeycomb structure according to claim 4, which further comprises a catalyst loaded onto at least one of each surface of the partition walls and each pore formed in the partition walls, and including titanium oxide as a main component and further including tungsten oxide and vanadium oxide. 